The present invention relates to piezoelectric ceramics and piezoelectric ceramic elements including the same. The present invention particularly relates to a piezoelectric ceramic adapted to a material for piezoelectric ceramic elements such as piezoelectric ceramic filters, piezoelectric ceramic resonators, and piezoelectric ceramic oscillators, and to a piezoelectric ceramic element including the piezoelectric ceramic.
Hitherto, piezoelectric ceramics containing lead zirconate titanate (Pb(TixZr1-x)O3) or lead titanate (PbTiO3) as a main component have been widely used for piezoelectric ceramic elements such as piezoelectric ceramic filters, piezoelectric ceramic resonators, and piezoelectric ceramic oscillators. Since piezoelectric ceramics containing lead zirconate titanate or lead titanate as a main component contain a large amount of lead, there is a problem in that the uniformity of product deteriorates because of the vaporization of lead oxide in the manufacturing process. In order to prevent the above problem, piezoelectric ceramics containing no lead or a small amount of lead are required. In view of environmental issues, piezoelectric ceramics containing less lead are preferable.
Since piezoelectric ceramics containing a bismuth layer compound such as CaBi4Ti4O15 as a main component contain no lead oxides, the piezoelectric ceramics receive attention as materials for solving the above problem.
When a piezoelectric ceramic containing a bismuth layer compound such as GaBi4Ti4O15 as a main component is used for a piezoelectric ceramic resonator, there is a problem in that the maximum value Qmax of the electrical quality factor Q (1/tan xcex4) in a frequency band, that is, at a frequency between the resonance frequency and the antiresonant frequency, is not sufficiently large for practical use.
It is an object of the present invention to provide a piezoelectric ceramic that contains a bismuth layer compound such as CaBi4Ti4O15 as a main component and no lead compounds or a small amount of lead, and is used for piezoelectric ceramic elements having a Qmax with a practicable value, and to provide a piezoelectric ceramic element including the piezoelectric ceramic.
The present invention provides a piezoelectric ceramic including a bismuth layer compound containing Ca, Bi, Ti, and O, as a main component, wherein the molar ratio of Ca to Bi to Ti in the bismuth layer compound, which is the main component, is represented by a:b:c and formulas 0.15xe2x89xa6a/c less than 0.25 and 3.5xe2x89xa6(2a+3b)/cxe2x89xa63.88 are satisfied.
The piezoelectric ceramic according to the present invention may contain a bivalent metal element except Ca or a trivalent metal element except Bi, wherein the molar ratio of the metal element to Ti in the main component is 0.1 or less (but more than 0) to 1. In this case, the bivalent metal element except Ca contained in the piezoelectric ceramic is at least one selected from the group consisting of Mg, Sr, Ba, and Pb. In this case, the trivalent metal element except Bi contained in the piezoelectric ceramic is at least one selected from the group consisting of Sc, Y, La, Ce, Pr, Nd, Sm, Gd, Dy, Er, and Yb.
The piezoelectric ceramic according to the present invention further contains Zr, wherein the molar ratio of Zr to Ti in the main component is less than 0.25 (but more than 0) to 1.
The piezoelectric ceramic according to the present invention further contains 1.5% or less (but more than 0) by weight of Mn on a MnCO3 basis.
The present invention provides a piezoelectric ceramic element having a piezoelectric ceramic including the piezoelectric ceramic according to the present invention and an electrode provided to the piezoelectric ceramic.
In the piezoelectric ceramic containing a bismuth layer compound containing Ca, Bi, Ti, and O, as a main component, the molar ratio of Ca to Bi to Ti in the bismuth layer compound, which is the main component, is represented by a:b:c and formulas 0.15xe2x89xa6a/c less than 0.25 and 3.5xe2x89xa6(2a+3b)/cxe2x89xa63.88 are satisfied. The reason for limiting the conditions to the above ranges is to obtain a practicable Qmax.
When the piezoelectric ceramic according to the present invention contains the bivalent metal element except Ca or the trivalent metal element except Bi, wherein the molar ratio of the metal element to Ti in the main component is 0.1 or less (but more than 0) to 1, the effects of the present invention are remarkable. The reason for limiting the molar ratio of the bivalent metal element except Ca or the molar ratio of the trivalent metal element except Bi to Ti in the main component to the ratio 0.1 or less (but more than 0) to 1 is as follows: when the ratio of the metal element to Ti exceeds the above value, the Qmax is smaller than that of a piezoelectric ceramic not containing the metal element; hence, it is unnecessary to have the metal element.
When the piezoelectric ceramic according to the present invention contains Zr, wherein the molar ratio of Zr to Ti in the main component is 0.25 or less (but more than 0) to 1, the effects of the present invention are remarkable. The reason for limiting the molar ratio of Zr to Ti to the ratio of 0.25 or less (but more than 0) to 1 is as follows: when the ratio of Zr to Ti exceeds the above value, the Qmax is smaller than that of the piezoelectric ceramic not containing Zr metal element; hence, it is unnecessary to have Zr.
When the piezoelectric ceramic according to the present invention contains 1.5% or less (but more than 0) by weight of Mn on a MnCO3 basis, the effects of the present invention are remarkable. The reason for limiting the content of Mn to 1.5% or less (but more than 0) by weight of Mn on a MnCO3 basis is as follows: when the Mn content exceeds the above value, the Qmax is smaller than that of the piezoelectric ceramic not containing Zr metal element; hence, it is unnecessary to have Mn.